1. Technical Field
The present invention relates generally to acoustic resonant systems, and more particularly, to a method of matching an acoustic driver to an acoustic load in an acoustic resonant system, and the system so formed.
2. Related Art
Acoustic resonant systems, such as high-frequency, xe2x80x9cStirling-stylexe2x80x9d pulse tube cryocoolers, are often powered by an acoustic driver such as a linear-motor-driven, resonant compressor. These compressors must be operated close to their resonant frequency in order to obtain high efficiency. Simply achieving resonant conditions at the compressor is insufficient, however, since several other matching conditions must also be met for a practical machine.
In general, for independently designed acoustic drivers and loads, only some of these matching conditions will be satisfied, resulting in inferior performance. Depending on which parameters are considered fixed, it is likely that the design frequency for the load will differ from the resonant frequency of the complete system, or full acoustic power will be delivered at a stroke other than rated stroke. If full acoustic power is delivered at less than rated stroke, excessive driver current will be required. If full rated power is delivered at more than rated stroke, the machine will be stroke-limited and never achieve its designed cooling capacity.
In view of the foregoing, there is a need in the art for a method for matching independently designed acoustic loads and acoustic drivers, and an acoustic system so formed.
Historically, designers have tried to make the combined acoustic driver and load as small as possible. In order to accomplish this, a stroke volume of the driver and any other volume between the driver and load has been minimized as much as possible. The invention, however provides, inter alia, a matching volume positioned between the acoustic driver and load that is substantially greater than a stroke volume of the driver. The matching volume is such that an operating resonant frequency substantially equal to a preferred operating frequency of the acoustic load is achieved. The invention, hence, allows for independent design of an acoustic driver and load. As an alternative, or in addition to the sizing of the matching volume, the stroke volume may be sized such that, in combination with a moving mass and a characteristic stiffness of the acoustic driver and a characteristic load impedance, a resulting pressure wave delivers a preferred input acoustic flow amplitude to the load when the acoustic driver is operating at the operating resonant frequency, the preferred stroke, and the preferred force amplitude.
A first aspect of the invention is directed to a method for matching an acoustic driver to an acoustic load in a resonant acoustic system, the acoustic driver including a moving mass, a characteristic stiffness, a preferred force amplitude, and a preferred stroke; the acoustic load including a characteristic load impedance, a preferred input acoustic flow amplitude, and a preferred operating frequency, the method comprising the steps of: a) providing a matching volume between, and in communication with, the acoustic driver and the acoustic load, the matching volume being substantially greater in size than a stroke volume of the acoustic driver; and b) sizing the matching volume such that, in combination with the moving mass, the characteristic stiffness of the acoustic driver and the characteristic load impedance, a resulting pressure wave produces an operating resonant frequency substantially equal to the preferred operating frequency of the load.
A second aspect of the invention is directed to a method for matching an acoustic driver to an acoustic load in a resonant acoustic system, the acoustic driver including a moving mass, a characteristic stiffness, a preferred force amplitude, and a preferred stroke; the acoustic load including a characteristic load impedance, a preferred input acoustic flow amplitude, and a preferred operating frequency, the method comprising the steps of: a) providing a matching volume between, and in communication with, the acoustic driver and the acoustic load, the matching volume being substantially greater in size than a stroke volume of the acoustic driver; and b) sizing the stroke volume of the acoustic driver such that, in combination with the moving mass, the characteristic stiffness of the acoustic driver and the characteristic load impedance, a resulting pressure wave delivers the preferred input acoustic flow amplitude to the load when the acoustic driver is operating approximately at: the operating resonant frequency, the preferred stroke, and the preferred force amplitude.
A third aspect of the invention provides a resonant acoustic system comprising: an acoustic driver including a piston, the driver having a first, stroke volume that provides space for a stroke of the piston; an acoustic load receiving an acoustic pressure wave from the driver; and a second volume between the driver and the load, the second volume being substantially greater in size than the first, stroke volume, wherein the second volume is sized such that an operating resonant frequency substantially equal to a preferred operating frequency of the acoustic load is achieved.
A fourth aspect of the invention is directed to a resonant acoustic system comprising: an acoustic driver including a piston, the driver having a first, stroke volume that provides space for a stroke of the piston; an acoustic load receiving an acoustic pressure wave from the driver; and a second volume between the driver and the load, the second volume being substantially greater in size than the first, stroke volume, wherein the stroke volume of the acoustic driver is sized such that, in combination with the moving mass, the characteristic stiffness of the acoustic driver and the characteristic load impedance, a resulting pressure wave delivers the preferred input acoustic flow amplitude to the load when the acoustic driver is operating approximately at: the operating resonant frequency, the preferred stroke, and the preferred force amplitude.
A fifth aspect of the invention is directed to a cryocooler comprising: a linear-motor driven compressor including a piston, the compressor having a first, stroke volume that provides space for a stroke of the piston; a pulse tube expander receiving an acoustic pressure wave from the compressor; and a second volume between the compressor and the expander, the second volume being substantially greater in size than the first, stroke volume, wherein the second volume is sized such that an operating resonant frequency substantially equal to a preferred operating frequency of the acoustic load is achieved.
A sixth aspect of the invention is directed to a cryocooler comprising: a linear-motor driven compressor including a piston, the compressor having a first, stroke volume that provides space for a stroke of the piston; a pulse tube expander receiving an acoustic pressure wave from the compressor; and a second volume between the compressor and the expander, the second volume being substantially greater in size than the first, stroke volume, wherein the stroke volume of the compressor is sized such that, in combination with the moving mass, the characteristic stiffness of the compressor and the characteristic load impedance, a resulting pressure wave delivers the preferred input acoustic flow amplitude to the expander when the compressor is operating approximately at: the operating resonant frequency, the preferred stroke, and the preferred force amplitude.
The foregoing and other features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention.